1. Field of the Invention
The present invention relates to a signal processor, and more particularly to an Hadamard transform coefficient predictor which is applied to compression and decompression of image signal processing, and suitably used by using Hadamard transform of 8 points or 8.times.8 points.
2. Description of the Related Art
There is known a system which uses Hadamard transform for an coding system of audio signals and video signals. According to a general coding system using Hadamard transform, an input signal is sectioned into plural blocks, Hadamard transform is performed on each block to calculate transform coefficients for the block, and then the transform coefficients are subjected to variable-length coding processing.
The 8-point Hadamard transform is defined by the following equation (1). The inverse Hadamard transform is also defined by the same equation (1) although the input/output is merely exchanged to each other. ##EQU1##
A transform coefficient y(0) of the lowest order of the Hadamard transform is a value which is proportional to the average value of the block, and it is hereinafter referred to as "DC coefficient". The transform coefficients other than the DC coefficient are generally referred to as "AC coefficients".
In Japanese Laid-open Patent Application No Hei-8-205160 which was filed by the inventor of this applicant, a method of calculating a bit pattern of a transform coefficient in a block is proposed as a conventional method using Hadamard transform. This publication discloses a procedure of calculating a bit pattern for 8-point Hadamard transform case and 8.times.8-point Hadamard transform case respectively by utilizing the in-block mutual relationship between lower-order bits of transform coefficients. The residual bit pattern is calculated from a half number of bits, for example, three bits of lower order for the 8-point transform case and six bits of lower order for the 8.times.8 Hadamard transform.
Further, according to a paper (1) (Takashi Mochizuki, "Improvement of Coding Efficiency of Hadamard Transform Reversible Coding based on Prediction of AC Coefficients", Lecture Papers D-218 of Information/System Society Convention of 1996 of The Institute of Electronics, Information and Communication Engineers, a method of predicting AC coefficients of a block at the center from the DC coefficients at the neighboring (surrounding) blocks on the assumption that an input signal varies smoothly is proposed by the inventor of this application. The above paper (1) handles the 8.times.8 Hadamard transform, and when the average values of nine blocks are arranged as shown in FIG. 9, the prediction values of transform coefficients y (u,v) in horizontal u-order and vertical v-order are shown in FIG. 10.
In FIG. 10, the transform coefficients b10, b01, b02, b20, b11, b12 b21, b22 indicates the following equation: EQU b10=Dw-De, EQU b01=Dn-Ds, EQU b20=(Dw-2Dc+De)/4, EQU b02=(Dn-2Dc+Ds)/4, EQU b11=(Dnw-Dne-Dsw+Dse)/8, EQU b21={(Dnw-2Dn+Dne)-(Dsw-2Ds+Dse)}/32 EQU b12={(Dnw-2Dw+Dsw)-(Dne-2De+Dse)}/32, EQU b22={(4Dc-2(Dn+Dw+De+Ds)+(Dnw+Dne+Dsw+Dse))/128
However, it is the present situation that no method of predicting the values of the transform coefficients in the block has been proposed. According to the method disclosed in Japanese-laid open patent application No. Hei-8-205160, the pattern of the lower-order bits of the transform coefficients can be calculated, however, no consideration is made on the higher-order bits. Further, according to the method disclosed in the paper (1), the AC coefficients of the center block cannot be predicted unless the DC coefficients of the neighboring blocks are known.